1. Field of the Invention
The invention relates to field of disk drives and more particularly to methods for writing servo tracks on magnetic hard disks.
2. Description of Related Art
Hard disk drives provide prerecorded tracking servo data on the data recording surfaces of their magnetic hard disks. This servo data typically comprises servo bursts spaced evenly along tracks. Data is recorded between the servo bursts. In most cases, servo bursts are approximately radially aligned, describing a small arc from the disk's ID to its OD. This radial alignment makes them look like arced spokes of the wheel. They form an arc when the servo data is written by a rotary actuator that describes an arc as it traverses between a disk's ID and its OD.
FIG. 1 illustrates a disk 10 having a number of servo data spokes 12. While there are eight illustrated in the figure, a disk drive disk will typically have hundreds of such servo data spokes spaced at even angles around disk. The number of such servo data spokes depends upon the track density. As a general rule, the greater the number of spokes, the higher the track density that can be employed in the disk drive. In many disk drives today, the servo data takes up approximately 11 percent of the total disk drive recording surface.
The servo data bursts may be written onto a disk's surface using a variety of techniques. The most common method is to write the servo onto the disk using the disk drive's own magnetic head controlled typically by an externally introduced picker that grasps the drive's rotary actuator pivot arm upon which the read/write head is mounted. An external mechanism incrementally moves the arm while other circuits command the disk drive to write the servo bursts.
Another common servo-writing method comprises writing servo bursts outside the disk drive on a so-called servo writer. In one form, the disks are already assembled onto the disk drive spindle but prior to the disk drive spindle/disk combination, also known as a hub/disk assembly (“HDA”), being assembled into the disk drive itself. In a second form, the disks are written on a separate spindle altogether and are later assembled onto the disk drive's spindle. This latter technique permits a large number of disks to be the mounted on the same servo writer. This technique reduces costs.
Another emerging technique is to imprint servo patterns on a disk using a so-called stamper. The stamper itself is mastered using a fixed e-beam directed to a turntable mounted on the end of a rotary actuator pivot arm. The turntable is rotated while arm pivots.
However, when a servo pattern is written outside the disc drive using a rotary actuator or pivot arm of some type, it introduces a problem that the actuator pivot arms of the servo writers are typically of a different length and arrangement than the actuator pivot arms of a disk drive. This causes the servo data to be written in a radial pattern or arc that is different from the arc described by the disk drive's rotary actuator pivot arm. This in turn introduces problems in a disk drive's servo system.
When the servo is written by the disk drive's own actuator, the arc that the servo data spokes describe has the same radius of curvature as the disk drive's actuator pivot arm. This is important from a drive's performance. When a drive performs a seek, it is preferred to have the servo data bursts in the next track(s) at a precisely known offset either from index or from the radial location of the servo bursts on the track from which the seek is taken. This enables for example so-called “just in time” seeking where the actuator is controlled so that it arrives in the destination track just in time to read the servo burst of the target track. If the servo burst has a timing offset, either “just in time” seeking would not be possible, or the calculations would have to include the timing offset. The latter alternative would increase the complexity of the “just in time” seeking algorithms and may require a table look up to determine the offsets, track by track. In either case, having random offsets in the servo data degrades servo performance and is undesirable.
It is therefore desirable to permit servo data to be written onto a disk drive using a servo data writer or stamper while reducing or eliminating the problems caused by the differences between the radius of curvature of a servo data writer's rotary actuators and the radius of curvature of the disk drive's rotary actuator.